Background Our aim was to determine the prevalence of tobacco smoking and e-cigarettes and cannabis use during pregnancy, whether these were influenced by ethnicity, and their relationship to perinatal outcomes. Methods A study was carried out in 4465 infants whose mothers delivered during 2017 and 2018. Self-reported maternal smoking, e-cigarette and cannabis use at booking were recorded. Outcome measures were birthweight and head circumference z-scores and admission to the neonatal intensive care unit (NICU). Results Two hundred and five women reported smoking cigarettes (4.7%), five were using e-cigarettes (0.11%) and 106 were using cannabis (2.43%). Women were most likely to smoke if young (15–19 years old) or from a mixed-race or White background. Cigarette smoking was associated with a lower mean z-score for birthweight (−0.587 vs. −0.064) and head circumference (−0.782 vs. −0.157) (both outcomes P < 0.0001). Young, mixed-race women were most likely to be both smoking and using cannabis during pregnancy and their infants had a lower birthweight mean z score (−0.989 vs. −0.587, P = 0.028) and head circumference z score (−1.33 vs. 0.782, P = 0.025) than cigarette use alone. Conclusion Young, mixed-race women were most likely to be both smoking and using cannabis during pregnancy and should be targeted for cessation programmes.